Relative humidity (RH) within concrete can be measured by performing in-situ testing at different depths within a concrete slab. An RH gradient exists within a slab of concrete that has its outer surface exposed; the RH increases as the depth into the slab increases. When the outer surface of the concrete slab is covered by a material that is close to impermeable, thereby preventing moisture from migrating out of the concrete surface, the RH gradient equalizes to an average RH throughout the slab that is about equal to the RH at 40% depth of an uncovered slab drying from the surface only (resting on a vapor retarder), or equal to the RH at 20% depth of an uncovered slab drying from both top and bottom.
A conventional practice for measuring RH in concrete includes forming a hole into a concrete slab at a certain depth, inserting a liner into the hole that will allow for sealing off the sidewalls of the concrete, placing of a seal at the top of the liner to seal off the internal environment, allowing for the equilibration of the RH of the internal environment, and then inserting a testing instrument into the liner to measure the RH. Most currently available instruments consist of a hole liner into which is inserted a probe that includes the sensor and a plastic enclosure. The probe is allowed to equilibrate, then taken out and moved to the next sleeve for the next measurement.
However, these currently available instruments result in undesirable problems relating to the thermal equilibrium of the sensing device and the concrete. Because RH is temperature dependent, it is therefore necessary that the RH sensing element be in thermal equilibrium with the environment being measured. With these currently available instruments, the probe portion that is inserted into the hole liner needs to come to thermal equilibrium with the concrete before an accurate measurement can be made. When a person handles the probe before insertion, heat is transferred into the probe, making the probe warmer and making the RH readings less accurate. Also, the ambient air is often times at a different temperature than the concrete environment to be measured, causing heat transfer across the probe. Even 1° F. of temperature change in the sensing element can cause inaccuracies in RH measurement of several percent.
In addition, some currently available instruments include a manual switch that is used by a person to take a measurement. The need to manually operate a switch can make the timing of the measurement long and uncertain.